Retroviruses and long terminal repeat (LTR) retrotransposons comprise a family of mobile elements that encode reverse transcriptase and copy their RNA genome into a cDNA that is integrated into the host genome. These prolific genomic parasites constitute a significant fraction of virtually all eukaryotic genomes. Their success as parasites relies on an array of host functions, since retroviruses and retrotransposons have small genomes and complex modes of replication. Host factors that are required for retroelement replication are potential targets for small molecules to treat AIDS, yet few have been characterized thus far. The family of active Ty1 LTR-retrotransposons in the yeast, S. cerevisiae, provides a unique model system to explore the eukaryotic host-retrotransposon relationship. Using a synthetic genetic array screen and secondary molecular screens, my lab has identified 48 retrotransposition host factors (RHFs) that are required for accumulation of Ty1 cDNA. Many of these RHFs are global regulators of translation and mRNA localization and turnover, including ribosomal protein paralogs, ribosome biogenesis factors, a paralog of translation initiation factor eIF4G, nonsense-mediated decay proteins Upf1-Upf3, the activator of decapping, Dhh1, the decapping holoenzyme, Dcp1/Dcp2 and the 5' to 3' exoribonuclease, Xrn1. The RNA genome of Ty1, like that of retroviruses, functions as a template for both translation of virus- like particle (VLP) proteins and for reverse transcription within VLPs. We have recently found that eIF4G1 and two ribosomal protein paralogs are required for accumulation of TyA, the major capsid protein, while Xrn1 influences Ty1 RNA packaging into VLPs. Moreover, we have found that Ty1 RNA is translationally repressed in sucrose-dense ribonucleoprotein particles and that TyA is associated with cytoplasmic mRNA processing bodies (P bodies), where translational regulators and mRNA decay factors are concentrated. We propose to examine the mechanism of translational regulation of Ty1 RNA and determine whether this regulation plays a role in partitioning Ty1 RNA between translation and packaging in VLPs. In addition, we will examine the role of mRNA decay proteins in the formation of VLPs that are functional for reverse transcription. The specific aims are: (1) to identify the steps in retrotransposition that are blocked in rhf mutants with reduced levels of Ty1 cDNA; (2) to examine the role of ribosomal protein paralogs, ribosome biogenesis factors and eIF4G1 in the translational regulation and localization of Ty1 RNA and protein; and, (3) to characterize Ty1 RNPs and determine whether mRNA decay factors influence Ty1 RNA or protein localization, RNA packaging in VLPs and/or reverse transcription. This project will elucidate specific mechanisms by which eukaryotic host factors promote the replication of retrotransposons and retroviruses.